The principal objective of this project is to establish the molecular basis of the dynamic properties of cell membranes. Of particular interest are the mechanism that regulate membrane assembly, and the principles that underlie the control of membrane permeability. The studies to date have focused on the membrane lipids, and their contributions to the properties of cell membranes. Our approach to membrane assembly has been to use equilibrium methods to describe all states that form spontaneously in a system consisting of hydrated phospholipids suspended in water. This approach has led to the finding of the spontaneous formation of a single lipid bilayer at the air/water surface in equilibrium with the dispersed phospholipid phase in water. The surface bilayer exists at a singularity in temperature. The temperature of surface bilayer formation is a function of the fatty acid composition of the phospholipid: the longer the hydrocarbon chain length the higher the temperature, the more fatty acid unsaturation the lower the temperature of surface bilayer formation. A variety of phospholipids have been shown to form surface bilayers: phosphatidyl ethanolamines, phosphatidylcholines, sphyngomyelin, phosphatidyl glycerols. Thus, surface bilayer formation appears to be a general property of membrane phospholipids. Permeability studies of the dimyristoyl phosphatidyl choline surface bilayer indicate that the surface bilayer acts as a permeability barrier within a very narrow temperature interval, of the order of less than 0.01 degree. The narrow temperature range of surface bilayer existence suggests that this state may act as a gating mechanism during membrane depolarization. Studies are in progress for comparing the temperature of growth of bacteria with the surface bilayer temperature of the membrane lipid extracts of these bacterial membranes. If there is a direct correspondence between the growth temperature and the surface bilayer temperature it is likely that the process of surface bilayer formation is utilized by the cell for lipid assembly.